1. Field of the Invention
The present invention relates to an electrolytic capacitor having a solid electrolyte layer and a method of manufacturing the same.
2. Description of the Related Art
In recent years, as one of electronic parts adapted for high frequency applications, an electrolytic capacitor is mounted on various electronic devices. For example, under the circumstance that digitization, miniaturization and speedup of electronic devices is being acceleratedly progressed, larger capacity and lower impedance of an electrolytic capacitor are demanded and, in addition, assurance of operation stability and operation reliability and longer life are also demanded.
A main part (capacitor element) of an electrolytic capacitor has, for example, a stacked structure in which an anode made of a valve metal, an oxide film (dielectric layer) formed by anodizing the surface layer of the anode, an electrolyte layer, and a cathode are stacked in this order.
The electrolytic capacitors are roughly divided into two kinds according to the kind of the electrolyte layer; a liquid electrolytic capacitor whose capacitor element includes an electrolyte layer (electrolyte) made of a liquid material, and having a conductive mechanism mainly using ionic conduction, and a solid electrolytic capacitor whose capacitor element includes an electrolyte layer (solid electrolyte layer) made of a solid material such as complex salt or conductive high polymer, and having a conductive mechanism mainly using electron conduction. When the two kinds of electrolytic capacitors are compared with each other from the viewpoint of stability of operating characteristics, for example, in the liquid electrolytic capacitor, the operating characteristics deteriorate with time due to leakage and evaporation of the electrolyte. In contrast, the deterioration with time in the operating characteristics due to leakage and evaporation of the electrolyte does not occur in the solid electrolytic capacitor. Consequently, as an electrolytic capacitor which can become the main stream in future, recently, in place of the liquid electrolytic capacitor, the solid electrolytic capacitor is being actively researched and developed. In the research process on the solid electrolytic capacitor, for example, in consideration of a series of operating characteristics such as the leak current characteristic, impedance characteristic, and heat resistance, the main part of the solid electrolyte layer is rapidly shifting from manganese dioxide or complex salt to a conductive high polymer of a conjugated system.
For the solid electrolytic capacitor, in particular, based on the technical background to be described below, to prevent destruction (including firing and burning) caused by heat generated at the time of short circuit, the function of increasing resistance as the temperature rises within a predetermined temperature range (so-called PTC (Positive Temperature Coefficient) function) is in demand.
Specifically, the solid electrolytic capacitor is mounted on various electronic devices (electronic circuits) as described above and has an advantage of generally low failure rate. However, when an overvoltage (voltage larger than rated voltage) or backward voltage (voltage whose sign of positive or negative is opposite) is applied due to a trouble on an electronic circuit and a dielectric layer is partially destroyed due to the overvoltage or backward voltage, the anode, solid electrolyte layer, and cathode are unintentionally made conductive, so that short circuit occurs in the solid electrolytic capacitor. In the case where short circuit occurs, when excess current (short-circuit current) flows in the solid electrolytic capacitor, the solid electrolytic capacitor generates heat and, in some cases, is destroyed by firing or burning caused by the heat generation.
As a measure to prevent destruction caused by heat generated at the time of short circuit in the solid electrolytic capacitor and also to prevent destruction of the solid electrolytic capacitor and circuit parts mounted on an electronic circuit, for example, a technique of mounting a fuse on the solid electrolytic capacitor may be employed. A solid electrolytic capacitor on which the fuse is mounted and having a configuration that a cathode and a cathode lead (lead for passing current) are electrically connected to each other via the fuse is known. In a solid electrolytic capacitor of this kind, when the fuse is blown due to heat generation at the time of short circuit, a circuit mechanism is interrupted, that is, a current path of excess current is interrupted, so that destruction of the solid electrolytic capacitor is prevented. However, the solid electrolytic capacitor using the fuse has some problems due to a structural factor and a mechanical factor of the fuse. First, when a fuse is mounted on a solid electrolytic capacitor, the structure of the solid electrolytic capacitor is complicated and is enlarged. Second, the mechanical strength of the fuse is low, that is, it is difficult to handle the fuse. When the process of manufacturing the solid electrolytic capacitor is complicated, the manufacture yield deteriorates. Third, in some cases, reliability of a solid electrolytic capacitor on which a fuse is mounted is low. More concretely, for example, when the periphery of a fuse is firmly covered with a mold resin, even if the fuse is blown due to heat generation at the time of short circuit, there is the possibility that the fuse is not completely blown due to the existence of the mold resin, so that the circuit mechanism is not therefore interrupted and the solid electrolytic capacitor may be destroyed. Therefore, a safety mechanism replacing the fuse is needed to increase the reliability of prevention of destruction of the solid electrolytic capacitors, and the PTC function as the safety mechanism is in demand.
Some modes of an electrolytic capacitor having the PTC function have been already proposed. Concretely, for example, an electrolytic capacitor in which PTC thermistors are disposed so as to face a capacitor element, and the PTC thermistors and the capacitor element are covered with a mold resin is known (refer to, for example, Japanese Utility Model Laid-Open No. H05-006826). In the electrolytic capacitor of this kind, a PTC thermistor is not provided as a safety mechanism replacing the fuse. As another example, an electrolytic capacitor having a configuration in which an anode (internal terminal) of a capacitor element and an anode lead (external terminal) are electrically connected to each other via a PTC thermistor (semiconductor ceramic layer) is known (refer to, for example, Japanese Utility Model Laid-Open No. H05-023529). Further, for example, an electrolytic capacitor having a configuration in which one of electrodes (external electrode) of a capacitor element and an electrode lead (metal terminal) are electrically connected to each other via the PTC thermister (an excess current/overheat protection device having the PTC function) is known (refer to, for example, Japanese Patent Laid-Open No. H11-176695). Generally, the PTC thermistor is electrically connected to a capacitor element by thermo compression bonding or a conducive adhesive.
For a solid electrolytic capacitor having the PTC function, there are various demands from the following viewpoints.
In a process of manufacturing a solid electrolytic capacitor having the PTC function, to increase productivity of the solid electrolytic capacitor, the solid electrolytic capacitor has to be manufactured as easy as possible. However, the conventional solid electrolytic capacitor manufacturing method has the following problem. By using the PTC function of the PTC thermistor, destruction of the solid electrolytic capacitor caused by heat generated at the time of short circuit is prevented. Since the PTC thermistor is connected to the capacitor element to give the PTC function to the solid electrolytic capacitor, the solid electrolytic capacitor manufacturing process is complicated and the number of manufacturing processes increases only by the amount corresponding to the PTC thermistor connecting process required. As a result, it is difficult to increase productivity of the electrolytic capacitor. Therefore, to increase the productivity of the electrolytic capacitor while preventing destruction caused by heat generated at the time of short circuit by using the PTC function, it is an urgent necessity to establish a technique capable of manufacturing the solid electrolytic capacitor having the PTC function as easy as possible. In particular, in the case of establishing the technique capable of manufacturing the solid electrolytic capacitor having the PTC function as easy as possible, it is also important to simplify the configuration of the solid electrolytic capacitor as much as possible in consideration of miniaturization of the solid electrolytic capacitor.
In the process of manufacturing the solid electrolytic capacitor having the PTC function, to assure productivity by increasing the manufacture yield of the solid electrolytic capacitor, it is necessary to stably manufacture the solid electrolytic capacitor as much as possible. In the conventional solid electrolytic capacitor manufacturing method, however, for example, when a PTC thermistor is thermo-compression-bonded to the capacitor element, the dielectric layer is easily damaged severely due to a mechanical factor (excessive external force applied to the capacitor element) at the time of thermo compression bonding, there is the possibility that the solid electrolytic capacitor is mechanically destroyed during manufacture. Different from destruction of the solid electrolytic capacitor caused by heat generated at the time of short circuit, the mechanical destruction of the solid electrolytic capacitor is fatal one and the basic structure itself of the solid electrolytic capacitor is damaged. Therefore, the mechanical destruction cannot be prevented by using the PTC function. When the solid electrolytic capacitor is mechanically destroyed during manufacture, naturally, the manufacture yield deteriorates and productivity of the solid electrolytic capacitor cannot be assured, so that it becomes difficult to stably manufacture the solid electrolytic capacitor. Therefore, to increase the productivity by increasing the manufacture yield of the solid electrolytic capacity while preventing destruction caused by heat generated at the time of short circuit by using the PTC function, it is an urgent necessity to establish a technique capable of manufacturing the solid electrolytic capacitor having the PTC function as stably as possible. In particular, in the case of establishing the technique capable of manufacturing the solid electrolytic capacitor having the PTC function as stably as possible, as described above, it is also important to manufacture the solid electrolytic capacitor as easily as possible in consideration of productivity of the solid electrolytic capacitor.
Further, to achieve higher performance of the solid electrolytic capacitor having the PTC function, it is necessary to reduce the resistance characteristic of the solid electrolytic capacitor as much as possible. However, in the conventional solid electrolytic capacitor manufacturing method, the resistance characteristic is not sufficiently low to improve the performance, so that there is room for improvement from the viewpoint of the resistance characteristic. More concretely, for example, in the case of connecting a PTC thermistor to a capacitor element to give the PTC function to the solid electrolytic capacitor, the resistance characteristic of the solid electrolytic capacitor increases only by the amount corresponding to the resistance of the PTC thermistor and the contact resistance between the PTC thermistor and the capacitor element. The resistance characteristic can increase to the degree that an adverse influence is given to the performance of the solid electrolytic capacitor. In the case of bonding the PTC thermistor to the capacitor element by using a conductive adhesive to give the PTC function to the solid electrolytic capacitor, an amount of increase in the resistance characteristic is often smaller than that in the case of thermo-compression-bonding the PTC thermistor to the capacitor element. Consequently, from the viewpoint of preventing the resistance characteristic of the solid electrolytic capacitor from increasing, it is preferable to use the bonding method using the conductive adhesive. However, when increasing demand for higher performance of the solid electrolytic capacitor is considered, it cannot be said that the resistance characteristic of the solid electrolytic capacitor obtained in the case of using simply a conductive adhesive is sufficient. Therefore, to achieve higher performance of the solid electrolytic capacitor while preventing destruction caused by heat generated at the time of short circuit by using the PTC function, it is also an urgent necessity to establish a technique capable of reducing the resistance characteristic of the solid electrolytic capacitor having the PTC function as much as possible.